leds: qti-flash: Add maximum available flash current prediction

Add support to calculate maximum current available for flash module.
Clients of flash LED makes use of this API qti_flash_led_prepare()
to query maximum available current for different flash LED channels.

Change-Id: I26988b1afb1e57a190aa25061189b71ca3f7ea95
Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>